Every grade is unique, even if the value of the grade repeats itself. Say a 90 grade for math today, and a 90 grade for math in past period are different. This means you have to go for ERC 721 standard, non-fungible token.

However, your idea of using tokens for grades is conceptually flawed, since grade (as a concept) should not be transferable because it becomes an attribute of a person (sort of). In Ethereum you can transfer tokens from one address to another and grades should not be transferable because it would look like a fraud.